Method for the preparation of plastic foam

ABSTRACT

THERMOPLASTIC FOAM IS PREPARED BY EXTRUSION AND INJECTION OF A FLUID BLOWING AGENT INTO THE HEAT PLASTIFIED MASS. THE IMPROVEMENT IS THE INCLUSION OF AN INTERFACIAL SURFACE GENERATOR; THAT IS, MOTIONLESS IN-LINE MIXER, WHOSE MIXING, UNDER CONDITIONS OF STREAM-LINE FLOW, CAN BE CONSIDERED INDEPENDENT OF THROUGHPUT. FOAMS OF REDUCED DENSITY AND INCREASED HOMOGENEITY ARE OBTAINED. LOWER POWER IS REQUIRED, TOGETHER WITH SIMPLIFIEED EQUIPMENT AND REDUCED MAINTENANCE THEREOF.   D R A W I N G

1973 M. D. BUCKNER 3,751,377

METHOD FOR THE PREPARATION OF PLASTIC FOAM Filed Aug. 19, 1971 6 U N v nN w c;

INVENTOR.

Marya/7 0. Eu ckn er HGA'NT United States Patent 3,751,377 METHOD FORTHE PREPARATION OF PLASTIC FOAM Morgan D. Buckner, Magnolia, Ark.,assignor to The Dow Chemical Company, Midland, Mich. Filed Aug. 19,1971, Ser. No. 173,310 Int. Cl. B29d 27/02; (108i 33/02, 47/10 US. Cl.260-25 E 5 Claims ABSTRACT OF THE DISCLOSURE Thermoplastic foams arewell known as are processes for the production thereof by the extrusionof thermoplastic materials which include a blowing or gas producingagent. One particularly desirable manner of producing such thermoplasticfoams or foamable materials by an extrusion process is to provide astream of heat plastified thermoplastic material and add or inject intothe stream a volatile fluid foaming agent, the fluid foaming agent beinggenerally a non-solvent for the polymer at the extrusion temperature,admixing the blowing agent with the heat plastified gel, bringing thegel to a desired extrusion temperature; that is, a temperature above orbelow the foaming temperature, depending upon whether a foamed productor a foamable product is desired. From a practical standpoint, such aprocess has been found to have limitations, particularly concerning thedensity of the foam that can be produced thereby. For example, with agiven thermoplastic resin such as polystyrene, one can readily extrude asmall round foamed rod, such a rod having a diameter of one inch to twoinches of very low density foam. If one attempts to employ the same orsimilar conditions and proportions of blowing agent, feed, temperaturesand the like to extrude a foam body of larger cross-section; forexample, a billet 12 inches in thickness and 24 inches in width, one canbe eminently unsuccessful; the product can warp, twist and distort.Often if one sections an extruded foam billet in a plane generallynormal to the direction of extrusion, there are regions of varyingdensity and varying cell size. Sometimes these occur in a radialpattern; at other times they occur in a generally transverse pattern.

A wide variety of equipment has been employed to prepare thermoplasticfoams for extrusion and representative equipment is described in thefollowing US. patents: 2,669,751; 2,753,595; 2,740,157; 3,151,192 and3,160,688. Other more complex extrusion processes are known. However themore complex the equipment becomes usually the more ditlicult themaintenance and the lower the degree of reliability that can be obtainedin day to day operation. Such maintenance increases rapidly as moreelements with moving parts are employed and oftentimes the powerrequirements per unit of output is increased.

In the preparation of synthetic foams, oftentimes it is desirable to addvarious modifying materials during their preparation including finelydivided materials such as calcium silicate, dyes, pigments, fireretardants and the like or other finely divided solid incompatiblematerials that function as nucleating agents; that is, agents whichaffect cell size and generally cause reduction thereof. Gen- 3,751,377Patented Aug. 7, 1973 erally in preparing foams, in order to obtaindesired nucleation and the like, minimal amounts of the additives arerequired. Such additives frequently are more expensive than thethermoplastic material employed to prepare the foam and it is desirablethat such additives be utilized at minimal levels. Oftentimes suchadditives while contributing a desirable effect also will contribute oneor more undesirable effects. Frequently it is beneficial to employ suchadditives at minimal concentration.

It would be desirable if there were available an improved method for thepreparation of synthetic resinous thermoplastic foams.

It would also be desirable if there were available an improved methodfor the preparation of synthetic resinous thermoplastic foams whichprovided an improved prodnot and products requiring lesser quantities ofadditives.

It would further be desirable if there were available a method for thepreparation of synthetic resinous foams which uitilzed simplified low.power requirement process and apparatus.

These benefits and other advantages in accordance with the presentinvention are achieved in a method for the preparation of thermoplasticsynthetic resinous foam wherein a volatile fluid foaming agent which isgenerally a non-solvent for the resin is admixed with heat plastifiedresin to form a flowable gel and extruded into a region of lowerpressure and temperature, the improvement which comprises passing theheat plastified resin and foaming agent through an interfacial surfacegenerator prior to extruding into a zone of lower pressure.

The method of the present invention is readily employed utilizingapparatus comprising in cooperative combination a source of a heatplastified synthetic resinous material, means to introduce a volatilefluid foaming agent into the heat plastified stream, an interfacialsurface generator in operative communication with the source, theinterfacial surface generator having an inlet end and an outlet end, theinlet end receiving the heat plastified resin, the discharge enddischarging heat plastified resin and foaming agent, a die defining adischarge opening of a desired configuration, the die being in 01;)-erative communication with the discharge end of the interfacial surfacegenerator.

By the term interfacial surface generator is meant an inline motionlessmixer, sometimes referred to as a static mixer or static pipe mixer,whose mixing mechanism is generally unrelated to the throughput when thethroughput is flowing in the region of streamline flow. Such mixers maybe considered as layering mixers wherein the flowing stream is dividedand two component parts reshaped and joined together in such a way thatthe interface between the original elements of the stream issubstantially increased. Such mixers are well known in the art and someof these mixers and their mode of operation are described in thefollowing US. patents: 3,051,542; 3,051,453; 3,195,- 865; 3,206,170;3,239,197; 3,286,992; 3,328,003; 3,358,- 749; 3,382,534; 3,394,924;3,404,869; 3,406,947 and 3,506,244.

Extrudable foamable polymers incorporating a volatile fluid foamingagent are well known in the art and are well known commercially. Suchpolymers include alkenyl aromatic resinous polymers. By the term alkenylaromatic resinous is meant a solid polymer of one or more polymerizablealkenyl aromatic compounds. The polymer or copolymer comprises, inchemically combined form, at least 50 percent by weight of at least onealkenyl aromatic compound having the general formula Ar-o=oH, wherein Arrepresents an aromatic hydrocarbon radical, or an aromatichalohydrocarbon radical of the benzene series, and R is hydrogen or themethyl radical. Examples of such alkenyl aromatic resins are the solidhomopolymers of styrene, a-methylstyrene, o-methylstyrene,mmethylstyrene, p-methylstyrene, ar-ethylstyrene, ar-vinylxylene,ar-chlorostyrene or ar-bromostyrene; the solid copolymers of two or moreof such alkenyl aromatic compounds with minor amounts of other readilypolymerizable olefinic compounds such as methylmethacrylate oracrylonitrile, etc.

Typical of such polymers are foamed polystyrene and foamablepolystyrene; styrene/acrylonitrile polymers; foamed styrene/maleicanhydride polymers. Other foamable polymers include polyolefins such asfoamed polyethylene; foamed polypropylene and foamed resinous copolymersof ethylene and propylene; polycarprolactam foam; foamed polymers ofnylon 66, condensation product of hexamethylenediamine and adipic acid;foamed ethylene vinyl acetate copolymers; foamed polyvinyl chloride,foamed vinylidene chloride/vinyl chloride; foamedpolymethylmethacrylate; foamed polyethylacrylate and the like.

The method of the present invention is applied with particular benefitto alkenyl aromatic resinous foams such as foamed and foamablepolystyrene and foamed and foamable styrene/acrylonitrile polymers aswell as rubberreinforced styrene polymers such as the impactpolystyrenes and rubber-reinforced acrylonitrile/butadiene/styrenepolymers.

Employing the method of the present invention, substantially improveduniformity of the extrude is obtained whether it be extruded as a foamor as a foamable resin. For polymers in the foamed form,non-uniformities are most readily determined by judging cell sizeuniformity. Unfoamed polymers usually are evaluated by determiningvolatile or blowing agent content across a cross-section of the extrude.

Further features and advantages of the present invention will becomemore apparent from the following specification taken in connection withthe drawing wherein:

FIG. 1 schematically depicts one embodiment of the invention and theprior art.

FIG. 2 schematically depicts an alternate embodiment of the invention.

In FIG. 1 there is schematically shown a foam plastic extrusionapparatus generally designated by the reference numeral which can beconveniently employed to illustrate the practice of the method of thepresent invention and the prior art. The apparatus 10 comprises incooperative combination an extruder 11 which is a source of heatplastified synthetic resinous material. A volatile fluid foaming agentsupply 12 is in operative combination with the extruder 11 and providesvolatile fluid foaming agent to heat plastified gel within the extruder.The extruder 11 has a discharge 13 which is in operative communicationwith a first gel stream or foamable polymer processing unit 15. The unit15 in turn discharges to a second processing unit 16. The unit 16 inturn discharges to a third processing unit 17 which in turn dischargesto a fourth processing unit 18. The unit 18 in turn discharges to anextrusion die 19. The heat plastified gel in turn is discharged as afoamed or foamable elongate shaped body 20. The function of the firsttwo processing units 15 and 1 6 usually is to provide adequate admixingof the blowing agent with the heat plastified polymer. As the blowingagent or volatile fluid foaming agent is generally a nonsolvent for thepolymer at extrusion temperatures, oftentimes such blowing agentincorporation is accomplished with substantial diificulty. Usually, thethird and fourth processing units such as the units 17 and 18 removeheat from the heat plastified gel and bring the material to a desiredextrusion temperature prior to discharge from the die.

Although four processing units are depicted in FIG. 1, the number may bemore or less, depending upon the particular design of the equipment. Insome instances, for example, foam extrusion is accomplished by attachingthe die directly to the extruder and accomplishing the desired heating,mixing and cooling within the single piece of apparatus. However, thebasic functions as depicted in FIG. 1 are retained.

One particularly beneficial manner of preparing plastic foams as setforth in the prior art is set forth in US. 2,669,751 where, in essence,the heat plastified gel containing the blowing agent is discharged to aprocessing unit which comprises one or more screw extruders or similardevices mounted in series to subject the gel to mixing and coolingconditions prior to extrusion from the die. Each such unit requires adrive usually consisting of an electric motor and gear reducer.

In accordance with the method of the present invention, the gelprocessing train; that is, the functions equivalent to the first andsecond processing units 15 and 16, are readily replaced with substantialbenefit by means of an interfacial surface generator; for example,interfacial surface generators as shown in US. Pats. 3,051,452; 3,051,-453 and 3,195,865. Surprisingly, when such a replacement is made,substantially improved results are obtained. The power requirements forthe extrusion train drop significantly in that the rotary mixers orextruders performing the mixing functions of the units 15 and 16 areeliminated without any significant increase in the power input requiredto the extruder 11. It must be appreciated, of course, that the choiceof an interfacial surface generator or interfacial surface generators toinstall as the processing units 15 and 16 must be made with due regardto pressure drop occurring during viscous flow of the material withinthe line. The selection of a suitable size of interfacial surfacegenerator for any particular polymer processing operation is well withinthe skill of the average designer of such equipment.

Advantageously, processing units such as the units 17 and 18 which serveto reduce the temperature of the heat plastified foamable stream todesired extrusion temperatures, may also be interfacial surfacegenerators. Because of the layering characteristics of the interfacialsurface generators, jacketing may be employed with great advantage toprovide a heat exchange jacket about the exterior of the interfacialsurface generator which will result in a very small temperature gradientbetween the various stream elements after the generation of at least,theoretically, many thousands of layers.

In FIG. 2 there is depicted an alternate embodiment of the extrusionapparatus for practice of the method of the present invention generallydesignated by the reference numeral 30. The embodiment 30 comprises incooperative combination a source of heat plastified synthetic resinousmaterial or an extruder 31 in operative communication with a firstinterfacial surface generator 32, a second interfacial surface generator33 and a third interfacial surface generator 34, the interfacial surfacegenerators in essence being connected in series; that is, wherein thefirst discharges to the second, the second to the third. The thirdinterfacial surface generator 34 discharges to a die 35 which may beconsidered as generally identical to the die 19 of FIG. 1. The source ofvolatile fluid foaming agent 37 is in selective operative communicationwith the source 31 by means of the conduit 39 having disposed therein avalve 41. The source 37 is also in selective communication with thefirst interfacial surface generator 32 through the conduit 39a havingdisposed therein a valve 41a.

In the embodiment of FIG. 2, volatile fluid foaming agent may be addeddirectly to the polymer source at the entry to the first interfacialsurface generator or may be added within the interfacial surfacegenerator; or alternately, depending upon the temperature of the source,a portion of the fluid foaming agent may be added prior to entry intothe interfacial surface generator and the remaining portion subsequentlyadded within the interfacial surface generator. "Ihe dual or multipleaddition is particularly advantageous wherein relatively largequantities of blowing agent are being added to the heat plastifiedpolymer. In the art of mixing employing interfacial surface generators,it is generally considered undesirable to utilize an interfacial surfacegenerator with materials having widely difiering viscosities due to theeffect of channeling and the like.

The following examples illustrate the manner in which the principles ofthe invention are applied but are not to be construed as limiting thescope of the invention.

EXAMPLE 1 Polystyrene foam is prepared employing as feed stock 100 partsby Weight of polystyrene and parts per 100 of a blowing agent which is amixture of 50 parts of dichlorodifiuoromethane and 50 parts of methylchloride. A 1% inch extruder is employed as a means to heat plastify thepolystyrene. The methyl chloride is introduced through a side injectionport positioned adjacent the discharge end of the screw and about of thedistance between the feed port and the discharge end of the screw. Thedischarge of the extruder is passed to an interfacial surface generatorhaving 28 mixing elements or stages having a flow pattern as describedin U.S. 3,406,947. The discharge of the interfacial surface generator isin operative combination with a die of appropriate size to provide afoamed strip about /2 inch in thickness and 12 inches in width. Theextruded foam obtained has a density of about 1.8

described in U.S. 3,406,947 is added to the extrusion train and receivespolymer directly from the extruder. The extrusion conditions are thenoptimized to determine the maximum amount of blowing agent which can beincorporated within the polymer and provide a uniform commerciallyacceptable polystyrene foam employing a feed rate of 100 pounds per hourof polystyrene, together with 0.03 part per hundred parts of polystyreneof magnesium oxide and 0.01 part per hundred parts of polystyrene ofindigo. Extrusion conditions are as follows: extruder pressure-1100pounds per square inch; die pressure540 pounds per square inch; extrudertemperature2l2 C.; foaming temperature-113 C. Ten parts per hundredparts of polystyrene of the 60/40 mixture ofmethylchloride/dichlorodifluoromethane blowing agent are used andcommercially acceptable foam is obtained having a density of 2.1 poundsper cubic foot.

EXAMPLE 3 A generally similar comparison is made employing a largerpolystyrene foam extrusion train wherein a 10 stage interfacial surfacegenerator having a flow pattern as described in U.S. 3,406,947, isinserted immediately after the extruder. A feed rate of 1800 pounds perhour is equal parts by weight of methylchloride anddichlorodifluoromethane and minor quantities of self-extinguishing andnucleating agents are employed in equal amounts in both cases. Theresults are set forth in the following table:

TABLE Extrude gel Foaming Extruder Die temperatempera- Blowing FoamConditions pressure 1 pressure 1 ture 1 ture 2 agent 8 density Withinterfacial surface generator 1, 180 610 215 114 14. 0 1. 6 Withoutinterfacial surface generator. 1, 100 580 220 115 11. 5 2. 0

1 Pounds per square inch. 1 Degrees C. I Parts per hundred.

pounds per cubic foot and when sectioned shows excellent cross-sectionaluniformity of cell size. For purposes of comparison, the interfacialsurface generators are replaced with a second 1% inch extruder, bothscrews rotating at the same speed as in the original experimentemploying the interfacial surface generators. A maximum of about 8 partsof blowing agent per 100 parts of polymer is the maximum amount ofblowing agent which can be employed without the formation of blow holesand large irregularities in the foam. In each instance, identicalpolymer and blowing agent mixtures are employed and conditions optimizedto obtain maximum production rates with uniform quality.

EXAMPLE 2 A polystyrene extrusion train utilizing a 2.5 inch extruderand rotary cooler similar to that depicted in U.S. 2,669,751 is operatedto optimize the extruded foam to provide a foam of generally uniform lowdensity and utilize a maximum quantity of blowing agent. The blowingagent is a mixture of 60 parts by weight methyl chloride and parts byweight of dichlorodifluoromethane. The operating conditions are asfollows: extruder pressure- 1420 pounds per square inch; die pressure670pounds per square inch; extruder temperature-208 C.; foamingtemperature-ll3 C.; 0.03 part per hundred parts of polystyrene ofmagnesium oxide and 0.04 part of indigo per hundred parts of polystyreneare employed at a fixed feed rate of 100 pounds per hour of polystyrene.The maximum amount of blowing agent which can be incorporated into theextruded foam is 8 parts per hundred without producing an unsatisfactoryfoam. The density of the foam is 2.6 pounds per cubic foot. A 10 stageinterfacial surface generator having a flow pattern as In a mannersimilar to the foregoing illustrations, the hereinbefore delineatedpolymers and blowing agents are employed to produce uniform qualitysynthetic resinous foams.

As is apparent from the foregoing specification, the present inventionis susceptible of being embodied with various alterations andmodifications which may differ particularly from those that have beendescribed in the preceding sepecification and description. For thisreason, it is to be fully understood that all of the foregoing isintended to be merely illustrative and is not to be constnued orinterpreted as being restrictive or otherwise limiting of the presentinvention.

What is claimed is:

1. A method for the processing of thermoplastic synthetic resin whereina volatile fluid foaming agent which is generally a non-solvent for theresin is admixed with heat plastified resin to form a flowable gel andextruded into a region of lower pressure and temperature, theimprovement which comprises passing the heat plastified resin andfoaming agent through an interfacial surface generator prior toextruding into a zone of lower pressure and obtaining foams of improvedhomogeneity.

2. The method of claim 1 wherein the foaming agent is introduced intothe heat plastified resin prior to entering the interfacial surfacegenerator.

3. The method of claim 1 wherein a portion of the foaming agent isintroduced into the heat plastified resin prior to entering theinterfacial surface generator and the remaining foaming agent isintroduced within the interfacial surface generator.

4. The method of claim 1 wherein the resin is an alkenyl aromatic resin.

5. The method of claim 1 including the step of mechanically admixing atleast a portion of the foaming 7 8 agent with the heat plastified resinprior to entry of the FOREIGN PATENTS heat plastified resin and foamingagent into the interfacial 1 202 127 12/1970 Great Britain. slurfacegenerator.

References Cited WILBERT J. BRIGGS, SR., Primary Examiner UNITED STATESPATENTS 5 2,669,751 2/1954 McCurdy et a1. 2602.5 E

3,368,008 2/1968 AZuma 26453 260-25 HB, HA, N, 93.5 A, 892; 26453; 425453 33 UNiTED STATES PATENT OFFIQE CERTIFICATE .OF CORRECTION I Patent 3751:37? Dated August 7,1973- Inventor(s) Morgan D, Buckner It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 2, line 55, the first patent number "3 ,051,542" Should be --3,O5,l, 1:52:"-. a

Signed and sealed this 5th day of November '1974.

(SEAL) Attest:

McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer. Commissioner ofPatents

